1. Field of the Invention
The present invention relates to a hologram recording/reproducing method for recording an interference fringe of two light beams onto a hologram recording medium and for reproducing data on the basis of light diffracted by the recorded interference fringe, and to a hologram device.
2. Description of the Related Art
In recent years, holographic technology has been developed rapidly towards practical application of a holographic memory, which is drawing much attention as a powerful candidate against optical discs of the next generation and the generation after that. Development of a holographic data storage system that performs recording and reproducing of large-volume data using the holographic technology has been proposed.
According to a hologram recording technique in related art, a recording operation is implemented by producing an interference fringe of reference light and signal light on a hologram recording medium and then recording the interference fringe on the hologram recording medium. In view of achieving practical application, a hologram recording device in related art may be problematic in that optical paths of the reference light and the signal light may change if the device vibrates during an exposure process or the interference fringe may change if the optical paths of the reference light and the signal light vibrate, thus inhibiting a proper recording operation. In a holographic memory for recording optical wave-front information, it is difficult to produce an interference fringe if the optical paths change by a tenth or more of the usable wavelength. In order to solve such problems, positioning control (servo) of a pickup unit having a high frequency response characteristic that can compensate for such vibration has been developed recently. However, unlike a typical optical recording technique where recording is performed in a bit-by-bit fashion, it is extremely difficult to implement positioning control in hologram recording while inhibiting aberration since holograms are generally for two-dimensional recording. Therefore, with hologram recording, simply increasing the frequency response characteristic of a pickup unit does not necessarily solve the abovementioned problems.
Meanwhile, if an error occurs in a typical optical disc, such as DVD (digital versatile disc) and BD (blue-ray disc), the error can be corrected by a signal processing technique or an error correcting technique. However, these optical discs are limited in that if an unexpected error occurs, at least the corresponding track entirely becomes defective. A typical data storage disc generally allows recording of one track at a time, which means that if such an error occurs, the whole disc may have to be discarded. This results in a waste of time and money for the recording media.
Hologram recording media have the same problems as current optical discs. This means that in a case where an error occurs, which may be not correctable by a signal processing technique or an error correcting technique, the hologram recording medium with the error may have to be discarded due to the same reason as above.
A typical example of a hologram multiplex-recording technique is an angular multiplexing technique in which an incident angle of reference light on a hologram recording medium is changed at high speed so as to record multiplex data onto a single book (for example, see Japanese Unexamined Patent Application Publication No. 2003-337524, p. 11, FIG. 5). Therefore, a large dynamic range may be necessary in hologram recording media, and in order to achieve this, an expensive material, such as a photopolymer, is used for the hologram recording media, which may lead to higher costs in comparison to typical optical discs. It is therefore highly desirable to prevent discarding of expensive media as a result of an error described above.